Various mechanisms have been used to enable tissue products, such as facial tissue, bath tissue, paper towels, sanitary napkins, and the like, to have high bulk and a soft feel. For example, one method that has been developed to form a soft tissue product is known as xe2x80x9cthrough-air dryingxe2x80x9d, which is a relatively non-compressive method of removing water from the web by passing hot air through the web until it is dry.
One particular method used to through-dry a web includes initially depositing an aqueous suspension of papermaking fibers onto the surface of an endless traveling foraminous forming fabric to form a wet web. Thereafter, the wet web is transferred to a transfer fabric traveling at a speed slower than the forming fabric, which is often referred to as xe2x80x9crush transferxe2x80x9d. After being transferred to the transfer fabric, the web is then transferred to a patterned through-drying fabric. The wet web is molded to the contours of the patterned through-drying fabric to increase the bulk of the web. Vacuum pressure can be used during transfer to draw the web onto the surface of the fabric. The pressure supplied by the vacuum is usually increased or decreased to vary the force with which the web is drawn onto the through-drying fabric to alter the degree of molding.
Nevertheless, using vacuum pressure to alter the degree of molding has significant limitations. Specifically, if the amount of vacuum pressure is too great, the web begins to form xe2x80x9cpinholesxe2x80x9d that can affect various properties (e.g., absorbency) of the resulting tissue product. Moreover, if the amount of vacuum pressure is too small, the web might not adequately adhere to the fabric. Further, high vacuum pressures can require a substantial amount of power. Also, in many instances, such as when using a highly textured fabric, it is occasionally not possible to fully hold the sheet against such fabric. Thus, previous methods for controlling the degree to which a web will mold to a through-drying fabric are severely limited.
As such, a need currently exists for better controlling the degree of molding during the formation of a tissue product.
In accordance with one embodiment of the present invention, a method of controlling the degree of molding of a paper web during formation of a tissue product is disclosed that includes providing a liquid furnish containing papermaking fibers. The furnish is deposited onto a foraminous surface to form a paper web. In one embodiment, once formed, the paper web may then optionally be transferred to a transfer fabric. A relative speed difference can exist between the foraminous surface and the transfer fabric to enhance the machine-direction stretch of the resulting paper web. While on the transfer fabric, the web may also be subjected to a variety of different treatments. For instance, in some embodiments, the web can be dewatered and/or applied with additional water.
The paper web is then transferred to a through-drying fabric that has a three-dimensional surface contour. The web can be transferred directly from the foraminous surface, from the transfer fabric, or from any other surface containing the web. Once transferred to the through-drying fabric, however, the web is deflected thereon using a certain pressure such that the web is substantially molded to the three-dimensional surface contour of the through-drying fabric. For example, in one embodiment, a negative pressure (e.g., vacuum) can be utilized to draw the web onto the surface contours of the through-drying fabric during transfer thereto.
In accordance with the present invention, the degree to which the paper web molds to the three-dimensional surface contour of the through-drying fabric (expressed in terms of caliper, cross-directional stretch, or combinations thereof) is controlled by a method that includes predetermining the degree to which the paper web molds to the three-dimensional surface contour of the through-drying fabric while at a first solids consistency and a certain deflection pressure. The degree of molding is then either increased or decreased by selectively adjusting the first solids consistency to a second solids consistency while holding the deflection pressure constant. For example, in one embodiment, the degree of molding is increased by decreasing the first solids consistency to a second solids consistency while holding the deflection pressure constant. This second solids consistency may, in such instances, be less than about 40%, in some embodiments between about 10% to about 34%, and in some embodiments, between about 15% to about 30%. In another embodiment, the degree of molding is decreased by increasing the first solids consistency to a second solids consistency while holding the deflection pressure constant. This second solids consistency may, in such instances, be greater than about 10%, in some embodiments between about 10% to about 34%, and in some embodiments, between about 15% to about 30%. Once transferred to the through-drying fabric, the web can then be substantially dried with a dryer, such as a through-air dryer.
Using the method of the present invention, it has been discovered that the degree of molding, as expressed in terms of caliper or CD stretch, can be controlled without having to change the deflection pressure. For example, in some embodiments, the caliper of the paper web can be increased or decreased up to about 30% from the predetermined caliper, while the cross-directional stretch can be increased or decreased at least about 30% from the predetermined cross-directional stretch.
Other features and aspects of the present invention are discussed in greater detail below.